It is normal to locate the catalytic converter under the floor of a vehicle at a substantial distance from the engine. As a result, when the engine is first started and the exhaust gas is relatively cool, the efficiency of the converter is reduced and some untreated exhaust gas flows straight out of the tailpipe. In those first few minutes of engine warm-up, it has been found that a substantial amount of tailpipe emissions can occur. As emission regulations are tightened by the Government, automotive engineers are convinced that one of the few areas available for obtaining emission improvement is during the initial cold start-up of the engine. As is well known, catalytic converters are very effective in removing pollutants when hot and therefore it is important to get the converter hot as quickly as possible. As a result, engineers are proposing the use of so called "warm-up" converters which are to be located as close as possible to the engine and be combined with the conventional catalytic converter downstream of the engine.
One form of catalytic converter that has been proposed as a warm-up converter can be seen in U.S. Pat. No. 5,575,980, issued in the name of Alan G. Turek on Nov. 19, 1996, and assigned to the assignee of this invention. The Turek patent discloses a converter that includes a perforated tube in the form of two sets of staggered slots. The tube is surrounded by a catalytic monolith composed of a series of alternating herringbone patterned foil sheets coated with a noble metal catalytic material. A valve is located in the tube downstream of the catalytic monolith and when the valve is in its closed position, the exhaust gas entering the tube passes through the slots in the tube and then flows radially through the monolith to catalytically react with the noble metal catalyst on the foil sheets. Once the exhaust gas is at or near the light-off or operational temperature of the downstream main catalytic converter, the valve is moved to the open position so that the exhaust gas flows entirely through the tube and is delivered in its heated state to the main catalytic converter.